1. Field of the Invention
This invention relates generally to the field of connecting to storage media such as disks and tapes and more particularly to the field of connecting to more SCSI storage devices than are physically available on a particular configuration or within a certain distance at a particular location.
2. Background
Many computer systems such as those using the UNIX.TM., MS/DOS.TM., Windows.TM., NT.TM., or Macintosh.TM. operating systems connect to mass storage devices over a parallel bus or connection known as a SCSI bus. Originally standing for Small Computer Systems Interface, SCSI is now synonymous with several ANSI standards for such connections--including SCSI-1, or SCSI-2, to Fast SCSI-2, Wide SCSI-2 or Fast-wide SCSI-2, and SCSI-3, among others. The SCSI standards describe not only the physical characteristics of the connection but also the protocol for sending and receiving data. One of the drawbacks of the SCSI standards is that they typically limit the number of SCSI "targets" (SCSI addresses on the bus-each target, in turn can include several devices which are addressed by logical unit numbers, "LUNs") which can be connected to a SCSI adapter or controller. For most computers there are also only a limited number of SCSI adapter or controller cards that can be connected to a host computer. Usually a SCSI adapter or controller can connect to only 8 or 16 targets and those targets must be physically located within 25 meters or a similarly small physical distance. This also tends to make it difficult to share SCSI devices between two or more computers.
It should be noted that SCSI-3 does allow a large number of LUN devices to be attached to each target, but it still limits the number of targets to 16 per adapter or controller, and still has physical distance limitations.
One attempt to overcome the distance limitations includes the use of repeaters or intermediate transmission devices, such as those described in U.S. Pat. No. 5,577,207 to Pauget et al., issued Nov. 19, 1996, entitled "System and Method for Connecting SCSI Units Spaced at Distance which is Greater than the Maximum Length Value of the SCSI Protocol." This is a hardware solution which requires the purchase of additional equipment. It attempts to overcome the distance limitations but not to address the limitations on the number of devices which can be attached.
SCSI devices have also been connected over local area network (LAN) networks, which include connections to printers and local servers. While not directly addressing any of the limitation problems, the Ethernet connection approaches usually involve some sort of hardware device such as a converter board which is included in the LAN and converts data from SCSI protocol format to Ethernet format, and vice versa. One such device designed to eliminate the need for a LAN server is described in U.S. Pat. No. 5,491,812, to Pisello et al., issued Feb. 13, 1996, entitled "System and Method for Ethernet to SCSI conversion." Such a device does not solve the problems on limitations on the number of targets. It also requires the purchase of additional hardware.
Fiber channel hardware technology overcomes many of these problems, by allowing a user to send SCSI protocol over a serial fiber optic bus that does not have the same limitations on number of connections, distances and sharing that standard SCSI buses do. However, fiber channel technology also requires the installation of new hardware connections between the SCSI devices and the computer, usually including new controllers or adapters.
These and other hardware solutions have been developed to overcome some or all of the SCSI limitations, but additional hardware is a key element in these solutions.
It is an object of this invention to provide a method for overcoming limitations on the number of SCSI devices which can be attached to a host computer.
It is another object of the present invention to provide a method for overcoming distance limitations on communications between a SCSI device and a host computer.